1. Field of the Invention
The present invention relates to thermoelectric materials to be employed for so-called thermoelectric conversion (i.e., direct energy conversion without use of any moving parts), including power generation on the basis of the Seebeck effect and electronic cooling on the basis of the Peltier effect. More particularly, the invention relates to thermoelectric materials comprising hybrid of organic polymer and inorganic thermoelectric materials for attaining, in combination, good moldability provided by the organic polymer and good thermoelectric characteristics provided by the inorganic thermoelectric materials. The invention also relates to a thermoelectric conversion device containing the materials and to a method for producing the materials.
2. Background Art
Thermoelectric conversion by use of a thermoelectric conversion materials; e.g., thermoelectric power generation or electronic cooling, finds utility in a simplified direct-energy-conversion apparatus having no mobile parts that generate vibration, noise, wear, etc.; having a simple, reliable structure; having a long service life; and facilitating maintenance. Thus, thermoelectric conversion is suitable for direct generation of DC power without combustion of a variety of fossil fuels or other sources and for temperature control without use of a cooling medium.
Characteristics of thermoelectric conversion materials are evaluated on the basis of thermoelectric power factor (TPF) and thermoelectric figure of merit (ZT), which are represented by the following formulas:
TPF=S2"sgr"xe2x80x83xe2x80x83[Formula 1]                    ZT        =                                                            S                2                            ⁢              σ                                      κ              xe2x80x2                                xc3x97          T                                    [                  Formula          ⁢                      xe2x80x83                    ⁢          2                ]            
wherein S represents the Seebeck coefficient; "sgr" represents electric conductivity; and xcexa represents thermal conductivity. Thermoelectric conversion materials desirably have a high ZT; i.e., a high Seebeck coefficient (S), high electric conductivity ("sgr"), and low thermal conductivity (xcexa).
For example, when employed for thermoelectric conversion such as thermoelectric power generation, thermoelectric conversion materials desirably have a thermoelectric figure of merit as high as ZT=0.02 or higher and to operate without variation for a long period of time under varying operation conditions. Mass production of thermoelectric power generators for use in vehicles or employing discharged heat gives rise to demand for thermoelectric conversion materials which have sufficiently high heat resistance and strength, particularly at high temperature, and resistance to deterioration in characteristics, as well as a method for producing the materials with high efficiency and at low cost.
Conventionally, PbTe or silicide materials including silicide compounds such as MSi2 (M: Cr, Mn, Fe, or Co) and mixtures thereof have been used to serve as the aforementioned thermoelectric conversion materials.
Sb compounds such as TSb3 (T: Co, Ir, or Ru) have also been used. For example, there has been disclosed thermoelectric materials which comprise materials containing CoSb3 as a predominant component and an impurity added for determination of conduction type (L. D. Dudkin and N. Kh. Abriko Sov, Soviet Physics Solid State Physics (1959) p. 126; B. N. Zobrinaand, L. D. Dudkin, Soviet Physics Solid State Physics (1960) p. 1668; and K. Matsubara, T. Iyanaga, T. Tsubouchi, K. Kishimoto, and T. Koyanagi, American Institute of Physics (1995) p. 226-229).
A variety of inorganic thermoelectric materials, including Bixe2x80x94(Te, Se) series (e.g., bismuth telluride); Sixe2x80x94Ge series; Pbxe2x80x94Te series; GeTexe2x80x94AgSbTe series; and (Ca, Sr, Bi)Co2O5 series, have been proposed and studied.
Some of the aforementioned inorganic thermoelectric materials have been proven to have excellent thermoelectric characteristics acceptable for practical use. However, these materials involve a drawback, in that they are difficult to process.
Japanese Patent Application Laid-Open (kokai) No. 8-32124 discloses a method for producing a thermoelectric conversion device including producing an ingot and cutting the ingot to thereby form a thermoelectric conversion device in the form of a rectangular prism. However, such an ingot is difficult to process, and material loss is significant. In addition, breaking and chipping during cutting is thought to lower the yield of the thermoelectric materials.
With regard to organic thermoelectric materials having good processability, polyaniline has been studied.
Another thermoelectric materials comprising polyaniline serving as organic thermoelectric materials and vanadium oxide has been proposed (E. Lazaro, M. Bhamidipati, M. Aldissi, and B. Dixon, AD Rep, p. 1-35 (1998)). Still another thermoelectric materials comprising polyaniline serving as organic thermoelectric materials and NaFeP (whiskers or nano-wires) has been proposed (J. Wang et al., 20th International Conference on Thermoelectrics, p. 352-355 (2001)).
However, these organic thermoelectric materials also involve a drawback, in that they have poor thermoelectric characteristics as compared with inorganic thermoelectric materials.
U.S. Pat. No. 5,973,050 discloses another thermoelectric materials based on organic thermoelectric materials in which metal (e.g., silver, gold, or platinum) in powder form is dispersed.
However, the organic thermoelectric materials disclosed in U.S. Pat. No. 5,973,050 has a Seebeck coefficient of p-type.
In general, thermoelectric materials employing organic thermoelectric materials exhibit p-type characteristics. When production of a thermoelectric device such as a Peltier device is contemplated, an n-type thermoelectric materials formed of the same materials as the counter p-type materials are required. Thus, provision of thermoelectric materials exhibiting the n-type thermoelectric characteristic is important.
The present inventors have conducted extensive studies in order to overcome the aforementioned drawbacks, and have found that hybridization of organic thermoelectric materials and inorganic thermoelectric materials through a specified method enables production of novel thermoelectric materials which exhibit the excellent processability of organic thermoelectric materials and the excellent thermoelectric characteristics of inorganic thermoelectric materials, and which may exhibit n-type thermoelectric characteristics. The present invention has been accomplished on the basis of this finding.
Thus, an object of the present invention is to provide thermoelectric materials having processability and excellent thermoelectric characteristics in combination and which can provide n-type thermoelectric characteristics in accordance with the nature of the employed inorganic thermoelectric materials. Another object of the invention is to provide a thermoelectric device employing the materials. Still another object of the invention is to provide a method for producing thermoelectric materials.
Accordingly, in a first aspect of the present invention, there are provided thermoelectric materials comprising an organic thermoelectric component and an inorganic thermoelectric component, wherein the organic thermoelectric component and the inorganic thermoelectric component are united in a dispersed state, the organic thermoelectric component being at least one species selected from among polyaniline and derivatives thereof; polypyrrole and derivatives thereof; polythiophene and derivatives thereof; polyphenylenevinylene derivatives; poly(p-phenylene) derivatives; polyacene derivatives; and copolymers thereof, and the inorganic thermoelectric component being at least one species selected from among Bixe2x80x94(Te, Se) series, Sixe2x80x94Ge series, Pbxe2x80x94Te series, GeTexe2x80x94AgSbTe series, (Co, Ir, RU)xe2x80x94Sb series, and (Ca, Sr, Bi)Co2O5 series.
The inorganic thermoelectric component may have a particle size of several hundreds xcexcm or less, whereby the thermoelectric materials are formed by dissolving the organic thermoelectric component in an organic solvent to thereby yield a solution; dispersing the inorganic thermoelectric component in the solution to thereby yield a dispersion liquid; and removing the organic solvent from the dispersion liquid.
The thermoelectric materials may be in the form of thin film.
The thermoelectric materials may form at least one layer in multilayer film comprising a plurality of stacked thin films.
The inorganic thermoelectric component in the form of microparticles may be surrounded by the organic thermoelectric component in the dispersed state.
In the thermoelectric materials the ratio by mol of the organic thermoelectric component to the inorganic thermoelectric component may be at least 1/99.
In the thermoelectric materials the ratio by mol of the organic thermoelectric component to the inorganic thermoelectric component may be 1/99 to 91/9.
The organic and inorganic thermoelectric components may be united through heat treatment.
The heat treatment may be performed at 50xc2x0 C. to 500xc2x0 C.
The thermoelectric materials may have a thermoelectric figure of merit (ZT) of at least 0.02.
The inorganic thermoelectric component may further contain a plasticizer, and may be united such that microparticles thereof cohere together.
The thermoelectric materials may contain the inorganic thermoelectric component in an amount of 10-70 mol based on 1 mol of the organic thermoelectric component.
The plasticizer may be an ionic liquid.
The thermoelectric materials may contain the plasticizer in an amount of 0.01-0.2 mol based on 1 mol of the organic thermoelectric component.
The inorganic thermoelectric component may have a particle size of 50 xcexcm or less.
The inorganic thermoelectric component may be treated with a titanate series or silane series surface treating agent.
In a second aspect of the present invention, there is provided a thermoelectric device employing the aforementioned thermoelectric materials.
In a third aspect of the present invention, there is provided a method for producing thermoelectric materials comprising:
dispersing an inorganic thermoelectric component in an organic thermoelectric component solution in which at least a portion of an organic thermoelectric component is dissolved, to thereby yield a dispersion; and
applying the dispersion to an object, to thereby form thereon thin film having one or more layers;
the organic thermoelectric component being at least one species selected from among polyaniline and derivatives thereof; polypyrrole and derivatives thereof; polythiophene and derivatives thereof; polyphenylenevinylene derivatives; poly(p-phenylene) derivatives; polyacene derivatives; and copolymers thereof, and the inorganic thermoelectric component being at least one species selected from among Bixe2x80x94(Te, Se) series, Sixe2x80x94Ge series, Pbxe2x80x94Te series, GeTexe2x80x94AgSbTe series, (Co, Ir, Ru)xe2x80x94Sb series, and (Ca, Sr, Bi)Co2O5 series.
The inorganic thermoelectric component may have a particle size of 50 xcexcm or less.
The thermoelectric materials may contain the inorganic thermoelectric component in an amount of 10-70 mol based on 1 mol of the organic thermoelectric component.
The thermoelectric materials may further contain a plasticizer.
The plasticizer may be an ionic liquid.
The thermoelectric materials may contain the plasticizer in an amount of 0.01-0.2 mol based on 1 mol of the organic thermoelectric component.
The dispersion may be applied through a method selected from among casting, spin-coating, and dipping.
According to the present invention, hybridization is attained in a dispersion state where microparticles of an inorganic thermoelectric component are surrounded by an organic thermoelectric component. Thus, microparticles of the inorganic thermoelectric component are united by the mediation of an organic polymer serving virtually as a binder, to thereby establish an electric conductivity. The thus-yielded thermoelectric materials have processability almost comparable to that of organic polymeric materials and the excellent thermoelectric characteristics of inorganic thermoelectric materials. Therefore, thermoelectric materials exhibiting n-type thermoelectric characteristics and having such processability can also be produced.